Apparatus for preventing damage due to the above-mentioned short circuit current utilizes, in the known systems, a rapidly acting switch which reacts to the above-mentioned short circuit current and disconnects the battery. This disconnection must be achieved very rapidly so that the product of the short circuit current and the time during which this current flows remains within acceptable bounds. Generally bimetal switches are used for this purpose. These very seldom achieve the required switching speed. Further, for this type of switch, the danger exists that after a number of incorrect battery connections the contacts become welded together so that no more protection exists. The same problems are encountered when an external voltage source such as a starter battery is connected to the circuit.